/Engine/Source/Runtime/Experimental/ChaosCore/Public/Chaos/Rotation.h
EpsilonSq is approximately the square of the angle below which we cannot calculate the axis. It needs to be "much greater" than square of the error in the quaternion values which is usually ~1e-4, so values around 1e-3^2 = 1e-6 or greater are about right.
Whether the axis was successfully calculated (true except for very small angles around or less than Epsilon).
bool ToAxisAndAngleSafe &40; TVector< FRealSingle, 3 > & OutAxis, FRealSingle & OutAngle, const TVector< FRealSingle, 3 > & DefaultAxis, FRealSingle EpsilionSq &41; const